The present invention relates to a semiconductor device and a method of producing the semiconductor device.
A variable capacitance diode is called a varactor in which a capacitance is changed according to a direct current voltage applied between electrodes thereof. The variable capacitance diode is used for controlling a frequency as a circuit element of a voltage control oscillator (VOC) in a phase locked loop (PLL). The variable capacitance diode in a semiconductor integrated circuit is generally produced with a process similar to those of MOS transistors. That is, a source electrode is connected to a drain electrode, and a gate oxide layer formed with a gate electrode in between generates a capacitance as a capacitor.
FIG. 2 is a schematic view showing a general configuration of a semiconductor integrated circuit with the variable capacitance diode. The semiconductor integrated circuit has a plurality of input terminals 1 for receiving external signals including clock signals CLK, and the input terminals 1 are connected to a logic circuit 3 via an input circuit 2. The logic circuit 3 performs a specific logical calculation process according to an external signal applied to the input terminals 1, and is formed of a combination of logic gates formed of several MOD transistors and the like.
The logic circuit 3 has VCO and PLL (not shown) synchronizing the clock signal CLK received from outside, so that an internal clock signal with a frequency different from that of the clock signal CLK is generated. A variable capacitance diode 4 is used in VCO as a capacitor of, for example, an LC resonant circuit formed of a coil and a capacitor. A variable direct current voltage is applied to a control electrode of the variable capacitance diode 4 for controlling an oscillating frequency. A result signal obtained in the login circuit 3 is sent to output terminals 6 via an output circuit 5.
The input circuit 2 protects the logic circuit 3 from a static surge voltage entering through the input terminals 1. The input circuit 2 has a protection diode connected between the input terminals 1, and a power source terminal and a ground terminal (not shown). The input circuit 2 also has a buffer amplifier for sending an input signal to the logic circuit 3. Similarly, the output circuit 5 has a buffer amplifier for protecting the logic circuit 3 from a static surge voltage entering through the output terminals 6.
The buffer amplifiers in the input circuit 2 and the output circuit 5 are formed of transistors with a gate oxide layer having a thickness larger than that of those in the logic circuit 3 for preventing breakdown due to a static surge voltage and the like. For example, a transistor in the logic circuit 3 may have a gate oxide layer with a thickness of 2.5 nm, while a transistor in the input circuit 2 or the output circuit 5 may have a gate oxide layer with a thickness of 5.0 nm. Similar to the transistor in the logic circuit 3, the variable capacitance diode 4 may have a gate oxide layer with a thickness of 2.5 nm. A pattern of the variable capacitance diode 4 is designed to have an area enough for obtaining a necessary capacitance according to a variable range of an oscillating frequency of VCO.
Although a purpose and a structure are different from those of the present invention, Patent Reference 1 discloses a method of producing a semiconductor device. A pair of varactor diodes in a same semiconductor is connected at short sides thereof, so that the semiconductor chip is thermally processed in a laterally elongated state to obtain a same capacitance for the varactor diodes.
Patent Reference 2 discloses a variable capacitance diode device, in which two variable diode elements with different effective areas are formed on a semiconductor substrate having a base substrate and an epitaxial layer. In the variable capacitance diode device, one variable diode element with a small effective area is used in a local oscillation circuit, and the other variable diode element with a large effective area is used in a high frequency circuit. An OSC signal and an RF signal with different levels per stage are overlapped with direct current voltages and applied, respectively. Accordingly, two C-V characteristics approach with each other, thereby reducing a tracking error.    Patent Reference 1: Japanese Patent Publication (Kokai) No. 2002-261298    Patent Reference 2: Japanese Patent Publication (Kokai) No. 2002-353469
In the semiconductor devices described above, when the oscillating frequency of VCO is changed, it is necessary to change an area of the variable capacitance diode 4. Accordingly, even though a circuit configuration is the same, it is necessary to change a circuit pattern according to the oscillating frequency.
In view of the problems described above, an object of the present invention is to provide a semiconductor device in which it is possible to arbitrarily change an oscillating frequency of VCO disposed therein without changing a circuit pattern.
Further objects and advantages of the invention will be apparent from the following description of the invention.